1. Field of the Invention
The present invention relates to adaptive array antennas and to a method of calibrating the same. The present invention is applicable and suitable for adaptive array antennas that control the directivity thereof when transmitting and receiving.
2. Description of the Related Art
In recent years, adaptive array antennas that can change the directivity thereof have been widely used. Adaptive array antennas have a plurality of antenna elements disposed in an array pattern. Each signal received by the corresponding antenna element is weighted by being multiplied by a corresponding weighting coefficient and all signals are then combined, thus capable of controlling the reception directivity pattern for all of the antenna elements. Also, transmission signals are each multiplied by the corresponding weighting coefficient used in receiving and are then transmitted from the respective antenna elements, so that the transmission signals are transmitted in a transmission directivity pattern that is the same as the reception directivity pattern. Consequently, in adaptive array antennas, desired waves can be spatially separated from interference waves, thus realizing SDMA (Space Division Multiple Access), or spatial multiple user access.
Adaptive array antennas need a plurality of antenna elements and the same number of transmission RF units and reception RF units as adaptive array elements. This causes the size of the overall apparatus to be larger. Therefore, adaptive array antennas are generally provided only in base stations.
In practice, fluctuations in characteristics and line length of the elements composing the transmission RF units and the reception RF units in adaptive array antennas cause a difference between the reception directivity pattern and the transmission directivity pattern, even if the weighting coefficients calculated for reception are used for transmitting.
In order to compensate for such fluctuations, correction value calculation processing, which is called calibration, is performed beforehand in adaptive array antennas. Accordingly, correction of the weighting coefficients by using the obtained correction values enables the reception directivity pattern to be made identical to the transmission directivity pattern.
One such method of calibrating adaptive array antennas is the so-called loopback method between antennas. In this method, one of a plurality of antenna elements composing an array antenna is set as a reference antenna. Calibration signals are sent from the reference antenna element, and are received by each of the other antenna elements. Then, the calibration signals are sent from each of the other antenna elements and are received by the reference antenna element. This is called loopback. Then, correction values (calibration factors) are calculated from transfer functions obtained at this time.
Ti: transfer function for i-th transmission RF unit
Rj: transfer function for j-th reception RF unit
Gij: transfer function for space between i-th antenna and j-th antenna
Under the conditions above, a transfer function “Sij” for the overall loopback can be expressed as in the following equation.Sij=TiGijRj  (1)
These “Ti” and “Rj” include transfer functions for antenna duplexers and the antennas themselves. A calibration factor “Hi” can be expressed as follows:Hi=Rj/Ti  (2)
“Qij”, which is a function that expresses the ratio of “Sij” to “Sji”, is adopted here.
                                                        Qij              =                            ⁢                              Sij                /                Sji                                                                                        =                            ⁢                              TiGijRj                /                TjGjiRi                                                                        (        3        )            
Furthermore, given that “Gij=Gji”, the following equation can be obtained.
                                                        Qij              =                            ⁢                              TiGijRj                /                TiGjiRi                                                                                        =                            ⁢                              TiRj                /                TjRi                                                                                        =                            ⁢                                                (                                      Rj                    /                    Tj                                    )                                ⁢                                  (                                      Ri                    /                    Ti                                    )                                                                                        (        4        )            
Assuming that the calibration factor “H1” for the first antenna, which is the reference antenna, is “1”, relative calibration factors for each antenna can be expressed as follows:
                                                        H2              =                            ⁢                              Q12H1                =                                  Q12                  =                                      S12                    /                    S21                                                                                                                          H3              =                            ⁢                              Q13H1                =                                  Q31                  =                                      S13                    /                    S31                                                                                                                          H4              =                            ⁢                              Q14H1                =                                  Q41                  =                                      S14                    /                    S41                                                                                                          (        5        )            
The weighting coefficients for transmitting are corrected using the calibration factors. Given that “Wri” represents the weighting coefficients for receiving, “Wti”, which represents the weighting coefficients for transmitting, can be expressed as in the following equation.Wti=Wri*Hi  (6)
Applying the transmission weights using the corrected weighting coefficients “Wti” enables the reception directivity pattern and the transmission directivity pattern of the adaptive array antenna to be made identical to each other.
When performing calibration in the base station by the loopback method between antennas described above, it is impossible to communicate with a mobile station Furthermore, if radio waves from a mobile station are received during calibration, errors may occur in the calibration. This causes the need for suspension of the operation of the communication system during calibration, thus causing a problem in that the overall communication system cannot be used.